In recent years, as a communication system providing an information service with high speed and large capacity, a WDM (Wavelength Division Multiplexing) optical communication system has been developed. In particular, as an interconnection device for an optical communication system, an optical wavelength routing device using an arrayed waveguide diffraction grating as an optical wavelength multiplexer/demultiplexer has been researched intensively.
For example, U.S. Pat. No. 5,412,744 discloses a frequency routing device having a spatially filtered optical grating for providing an increased passband width, which has a spectral efficiency maximized by providing a relatively wide passband and a relatively narrow channel spacing for a given cross talk level.
As a structure of an optical wavelength routing device using an arrayed waveguide, a structure comprising an arrayed waveguide provided between an input slab waveguide and an output slab waveguide has been known.
FIGS. 1A to 1C show a structure of a conventional optical wavelength routing device composed of an arrayed waveguide diffraction grating type optical multiplexer/demultiplexer, wherein FIG. 1A is a schematic view of the conventional optical wavelength routing device.
As shown in FIG. 1A, the optical wavelength routing device comprises a substrate 3, input waveguides 1 having input ports 7 with a total number M, an input slab waveguide 4 coupled to the input waveguides 1, which radiates a light supplied from the input waveguides 1 with predetermined angles by diffraction, a diffraction grating type arrayed waveguide 5 composed of a plurality of waveguides 8 and coupled to an output side of the input slab waveguide 4, which transmits lights supplied from the input slab waveguide 4 through the waveguides 8, an output slab waveguide 6 coupled to an output side of the arrayed waveguide 5, which focuses the radiated lights supplied from the arrayed waveguide 5 with predetermined angles, and output waveguides 2 having output ports 9 with a total number N and being coupled to an output side of the output slab waveguide 6.
Herein, the input and output waveguides land 2, the input and output slab waveguides 4 and 6, and the arrayed waveguide 5 are formed on the substrate 3. The light (optical signal) supplied to the input waveguides 1 is firstly radiated with the predetermined angles at the input slab waveguide 4 by diffraction, then transmitted through a plurality of the waveguides 8 of the arrayed waveguide 5. The radiated lights are focused at the output slab waveguide 6 and finally emitted from the output waveguides 2.
FIGS. 1B and 1C show a light input side and a light output side of the optical wavelength routing device, respectively. In FIG. 1B, the input ports 7 with the total number M of the input waveguides 1 are numbered (1_1) to (1_M), respectively, and an emitting angle of a light emitted from one of the input waveguides 1 is expressed as .PHI.. In FIG. 1C, the output ports 9 with the total number N of the output waveguides 2 are numbered (2_1) to (2_N), respectively, and a focusing angle of the light output from respective waveguides 8 of the arrayed waveguide 5 is expressed as .theta.. Both of the emitting angle .PHI. and the focusing angle .theta. are angles formed by respective optical axes of the lights relative to a central line O.
FIG. 2 shows an example of a wavelength routing function in an ideal optical wavelength routing device, wherein the total number of the input ports is M and the total number of the output ports is N.
In this optical wavelength routing device, when wavelength division multiplexed optical signals having different wavelengths .lambda..sub.1, .lambda..sub.2 - - - .lambda..sub.N are input into the first input port, optical signals each having one of wavelengths .lambda..sub.1, .lambda..sub.2 - - - .lambda..sub.N are output from the first to Nth output ports, respectively. Further, when the wavelength division multiplexed optical signals having different wavelengths .lambda..sub.1, .lambda..sub.2, - - - .lambda..sub.N are input into the second input port, the optical signals each having one of wavelengths .lambda..sub.2, .lambda..sub.3, - - - .lambda..sub.N, .lambda..sub.1 are output from the first to Nth output ports, respectively. Still further, when the wavelength division multiplexed optical signals having different wavelengths .lambda..sub.1, .lambda..sub.2, - - - .lambda..sub.N are input into the third input port, the optical signals each having one of wavelengths .lambda..sub.3, .lambda..sub.4, - - - .lambda..sub.N, .lambda..sub.1, .lambda..sub.2 are output in order from the first to Nth output ports, respectively. As described above, the ideal optical wavelength routing device has a routing function in which the wavelengths of the optical signals output from the output ports are circulated in accordance with the selection of an input port from the input ports.
Next, a wavelength routing function of the above explained conventional optical wavelength routing device composed of the arrayed waveguide diffraction grating type optical multiplexer/demultiplexer will be explained.
When a light having a wavelength .lambda. is input to each of input ports 7 of input waveguides 1, a relation of the wavelength .lambda. of the light, an emitting angle .PHI. at an input slab waveguide 4 and a focusing angle .theta. at an output slab waveguide 6 is given by a following formula (1): EQU .lambda.={n.sub.s (.lambda.)d/m}.multidot.(sin.PHI.+sin.theta.)+n.sub.eff (.lambda.).DELTA.L/m (1)
wherein .DELTA.L is a waveguide length difference of two adjacent waveguides 8 of an arrayed waveguide 5, d is an interval between central axes of the two adjacent waveguides 8 at coupling portions with the input slab waveguide 4 and the output slab waveguides 6, n.sub.eff (.lambda.) is an equivalent refractive index of the arrayed waveguide 5 for a propagated light having a wavelength .lambda., n.sub.s (.lambda.) is an equivalent refractive index of the input slab waveguide 4 and the output slab waveguide 6 for a propagated light having a wavelength .lambda., and m is a diffraction order number.
Accordingly, by aligning each of the input ports 7 of the input waveguides 1 and each of the output ports 9 of the output waveguides 2 with the emitting angle .PHI. and the focusing angle .theta., respectively, a light having a central wavelength .lambda. given by the formula (1) is output from each of the output port 9 of the output waveguides 2. Namely, the emitting angle .PHI. is a locating angle for locating the input waveguides 1 and the focusing angle .theta. is a locating angle for locating the output waveguides 2.
In this structure, when a wavelength range of the transmitting light is sufficiently narrow and respective ranges of the locating angles for locating the input waveguides 1 and the output waveguides 2 are sufficiently narrow, it is possible to approximate n.sub.eff (.lambda.), n.sub.s (.lambda.), sin.PHI., and sin.theta. to n.sub.eff, n.sub.s, .PHI., and .theta., respectively. Accordingly, the relation between the wavelength .lambda. of the transmitting light and the locating angles .PHI. and .theta. can be linear functions.
FIG. 3 shows a relation between the wavelength .lambda. of the transmitting light and the focusing angle .theta. , when the emitting angle .PHI. is kept to be constant, wherein a range (3-a) of light wavelengths .lambda..sub.1 to .lambda..sub.N indicates the wavelength range of the transmitting light, a curve (3-b) indicates a curve of a diffraction order number m, and a curve (3-c) indicates a curve of a diffraction order number m+1. As shown in FIG. 3, when a transmitting light (a WDM optical signal) is input into one of input ports 7 numbered (1-1), plural lights each having one of wavelengths .lambda..sub.1, .lambda..sub.2, - - - .lambda..sub.N are diffracted by mth order diffraction and output from respective output ports 9 numbered (2_1) to (2_N), each of which corresponds to each of focusing positions for focusing angles .theta..sub.1, .theta..sub.2 - - - .theta..sub.N.
Next, angles .theta..sub.N, .theta..sub.N+1, .theta..sub.N+2 - - - , are presumed, each of which has a predetermined angle interval equal to an angle interval of the focusing angles .theta..sub.1, .theta..sub.2, - - - .theta..sub.N. According to the formula (1), variations of the wavelength .lambda. can be equalized in both cases of varying the locating angles .PHI. and .theta., respectively, so that the influences of a varied amount .DELTA..PHI. of the emitting angle .PHI. and a varied amount .DELTA..theta. of the focusing angle .theta. on the wavelength .lambda. can be equalized. Accordingly, when .DELTA..PHI. and .DELTA..theta. are provided as a locating interval for each of two adjacent input ports 7 and a locating interval for each of two adjacent output ports 9, respectively, by replacing .DELTA..PHI. with .DELTA..theta., a light input into one of input ports 7 numbered (1_i) is output from respective output ports 9 numbered (2_1) to (2_N) (each of which actually corresponds to each of the focusing angles .theta..sub.1, .theta..sub.2, - - - .theta..sub.N) to have the same wavelengths to those corresponding to the focusing angles .theta..sub.i - - - .theta..sub.N+i-1 of range (3-d) for a light input into one of input ports 7 numbered (1_i).
Herein, when a diffraction order number m is determined such that a wavelength .lambda..sub.m+1 indicated by coordinates-value (3-e), in which a diffraction order number is m+1 for a focusing angle .theta..sub.N+1, has an approximate value of a wavelength .lambda..sub.1, the wavelength .lambda..sub.m+1 is given by a wavelength .lambda..sub.m, in which a diffraction order number is m for a focusing angle .theta..sub.N+1, in accordance with a following formula (2): EQU .lambda..sub.m+1 ={n.sub.eff (.lambda..sub.m+1)/n.sub.eff (.lambda..sub.m)}.multidot.{m/(m+1)}.multidot..lambda..sub.m (2)
wherein .lambda..sub.m =.DELTA..sub.N +.lambda..sub.1, and it is assumed that n.sub.s (.lambda.) can approximate to n.sub.eff (.lambda.)
In the conventional optical wavelength routing device, by determining the diffraction order number m as described above, the coordinates-value (3_e) indicating a value of the wavelength .lambda. for the focusing angle .theta..sub.N+1 can approximate to the coordinates-value (3-f) indicating a value of the wavelength .lambda..sub.1 for the focusing angle .theta..sub.N+1. When a light is input to one of the input ports 7 numbered (1_i) (wherein i .gtoreq.2), the light is output from respective output ports 9 numbered (2_(N-i+2)) to (2_N), each of which corresponds to each of the focusing angles .theta..sub.N+1 - - - .theta..sub.N+i-1. Since respective wavelengths of the output light have approximate values of .lambda..sub.1, - - - .lambda..sub.i-1 by the (m+1)th order diffraction, a function approximate to a predetermined routing function can be obtained.
According to the conventional optical wavelength routing device, however, when respective output ports with the total number N are arranged such that the light is input into a predetermined input port, and plural lights having predetermined central wavelengths are output from the respective output ports with the total number N, the equation shown as the formula (1) is not completely a linear function. Further, even if the curve (3-b) indicating the diffraction order number m of FIG. 3 is translated in the .theta. axis direction (right direction of FIG. 3), the curve (3-b) does not coincide completely with the curve (3-c) indicating the diffraction order number m+1. Although the curves (3-b) and (3-c) are made approximate to linear functions, respectively, inclinations of the linear functions approximate to the respective curves (3-b) and (3-c) are different to each other. Further, since m is an integer as shown in the formula (2), it is impossible to design the device such that a value of .lambda..sub.m+1 should be always equal to a value of .lambda..sub.1 except a particular value of .lambda..sub.1, so that the coordinates-value (3-e) cannot coincide with the coordinates-value (3-f) shown in FIG. 3. Accordingly, in the conventional optical wavelength routing device, there is a disadvantage in that, when a light is input into an input port other than aforementioned predetermined input port, the light having a central wavelength which is deviated from a predetermined central wavelength is output from each of the output ports.
FIG. 4 shows a deviation of central wavelengths in a conventional optical wavelength routing device. In FIG. 4, there is shown a relation between a focusing angle .theta. and a wavelength .lambda. in the conventional optical wavelength routing device, which is designed for satisfying following conditions. When a light is input into one of the input ports 7 of the input waveguides 1 numbered (1_1), plural lights each having one of central wavelengths .lambda..sub.1, .lambda..sub.2, - - - .lambda..sub.N are diffracted by mth order diffraction and output from respective output ports 9 numbered (2_1) to (2_N) . Further, when a light is input into one of the other input ports 7 of the input waveguides 1 numbered (1_2) to (1_M), plural lights each having one of central wavelengths .lambda..sub.2, - - - .lambda..sub.M are output from one of the output ports 9 numbered (2_1).
Herein, if a curve indicating a diffraction order number m+1 coincides with a chain line (4-a) indicating a desired wavelength, it is possible to obtain a routing function, in which a light having a predetermined central wavelength is output from each of output ports 9. However, according to the conventional optical wavelength routing device, a curve indicating the diffraction order number m+1 is actually expressed as a solid line (4-b). As shown in FIG. 4, in the conventional optical wavelength routing device, when a light is input into one of the input ports 7 numbered (1_M) and output from one of the output ports 9 numbered (2_N), an absolute value of a central wavelength difference, which is obtained by subtracting a predetermined central wavelength from a central wavelength of the light actually output from the output port, is expressed as .delta..lambda.. The value of .delta..lambda. is a maximum value of the absolute values of all the central wavelength differences shown in FIG. 4, which is called as a maximum central wavelength difference.
For example, where routing operation is conducted for sixteen lights having wavelengths .lambda..sub.1 =1540 nm to .lambda..sub.16 =1552 nm with a channel interval of 0.8 nm by using a 12.times.16 channel optical wavelength routing device comprising twelve input ports 7 and sixteen output ports 9, an arrayed waveguide 5, an input slab waveguide 4, an output slab waveguide 6, input waveguides 1, and output waveguides 2 are designed so as to satisfy following conditions. In this conventional optical wavelength routing device, a light input is supplied to one of the input ports 7 of the input waveguides 1 numbered (1_1) and plural lights each having one of central wavelengths .lambda..sub.1, .lambda..sub.2, - - - .lambda..sub.16 are output from respective output ports 9 numbered (2_1) to (2_16). Further, a light is input into one of the other input ports 7 of the input waveguides 1 numbered (1_2) to (1_12), and plural lights each having one of central wavelengths .lambda..sub.2, - - - .lambda..sub.12 are output from one of the output ports 9 numbered (2_1).
FIG. 5 shows calculated values of central wavelength differences in the conventional optical wavelength routing device made of silica like material, which is designed as described above. As shown in FIG. 5, according to this conventional optical wavelength routing device, there is a disadvantage in that the central wavelength differences are occurred. As understood from this graph, the light input into one of the input ports 7 numbered (1_12) and output from one of the output ports 9 numbered (2_16) has a maximum central wavelength difference .delta..lambda.=0.0987 nm.